Electrocardiogram (ECG) signal monitoring is important for understanding a patient's heart condition. Since a patient's skin exhibits voltage differences due to the muscle contractions of the heart, ECG signal acquisition involves strategically placing several electrode sensors on a patient and then evaluating the electrical signals from the sensors. A typical sensor includes a metal contact with conductive gel and attaches to the patient's skin with an adhesive bandage. The sensor generates sensor signals indicative of the skin voltages that represent the heart contractions. These sensor signals are fed to a recorder which may store them or print them out on a roll of paper.
ECG signal acquisition hardware designers are faced with problems of uncertain skin/electrode resistance and undesired noise induced by neighboring electromagnetic fields. One problem arises because skin signal voltage is on the order of 1 mV and must be amplified approximately 1000 .times. before processing. This small signal is susceptible to stray interference effects. The conventional approach to handling these problems is to invoke a rigorous skin preparation - including shaving skin, abrasion, and cleaning with isopropyl alcohol - and to limit the signal bandwidth to a span of 0.05 to 40 Hz to avoid 60 and 120 Hz interference.
Ambulatory out-patient monitors have these problems and more. Patients do not always follow a physician's instructions on scrubbing the skin and properly applying the sensors. Often, this causes noise that results in poor signal quality and lost data. Also, monitoring a patient's ECG while outside the hospital or physician's office may involve recording the ECG over long periods of time, which requires large data memory. Often, this large amount of data requires that the patient personally visit the hospital or office for the ECG data to be analyzed. Moreover, few devices allow the patient to identify when a cardiac episode occurred, permitting the doctor to analyze that specific ECG segment without reviewing the entire data record.
A monitor that can improve the signal-to-noise ratio of the heartbeat signal and that can efficiently record cardiac episodes would be helpful in diagnosing and correcting a patient's cardiac problem.